figure. They are quite large, and consist of a central projection of mucous membrane, surrounded by a wall, or circumvallation, from which they derive their name.

The Taste Beakers, supposed to be the true organs of taste, are flasklike bodies, ovoid in form, about of an inch in length, situated in the epithelial covering of the mucous membrane, on the circumvallate papillæ. They consist of a number of fusiform, narrow cells, and curved so as to form the walls of this flask-like body; in the interior are elongated cells, with large, clear nuclei, the taste cells.

Nerves of Taste.—The chorda tympani nerve, a branch of the facial, after leaving the cavity of the tympanum, joins the third division of the fifth nerve between the two pterygoid muscles, and then passes forward in the lingual branches, to be distributed to the mucous membrane of the anterior two-thirds of the tongue. Division or disease of this nerve is followed by

a loss of taste in the part to which it is distributed.

The glosso-pharyngeal enters the tongue at the posterior border of the hyo-glossus muscle, and is distributed to the mucous membrane of the base and sides of the tongue, fauces, etc.

The lingual branch of the trifacial nerve endows the tongue with general sensibility; the hypoglossal endows it with motion.

The nerves of taste in the superficial layer of the mucous membrane form a fine plexus, from which branches pass to the epithelium and penetrate it; others enter the taste beakers, and are directly connected with the taste cells.

The seat of the sense of taste has been shown by experiment to be the whole of the mucous membrane over the dorsum of the tongue, soft palate, fauces, and upper part of the pharynx.

The Sense of Taste enables us to distinguish the savor of substances introduced into the mouth, which is different from tactile sensibility. The sapid quality of substances appreciated by the tongue are designated as bitter, sweet, alkaline, sour, salt, etc.

The Essential Conditions for the production of the impressions of taste are (1) a state of solubility of the food; (2) a free secretion of the saliva, and (3) active movements on the part of the tongue, exciting pressure against the roof of the mouth, gums, etc., thus aiding the solution of various articles and their osmosis into the lingual papillæ. Sapid substances, when in a state of solution, pass into the interior of the taste beakers, and come into contact, through the medium of the taste cells, with the terminal filaments of the gustatory nerves.

THE SENSE OF SMELL.

The Sense of Smell is located in the mucous membrane lining the upper part of the nasal cavity, in which the olfactory nerves are distributed.

The Nasal Fossæ are two cavities, irregular in shape, separated by the vomer, the perpendicular plate of the ethmoid bone, and the triangular cartilage. They open anteriorly and posteriorly by the anterior and posterior nares, the latter communicating with the pharynx. They are lined by mucous membrane, of which the only portion capable of receiving odorous impressions is the part lining the upper one-third of the fossæ.

The Olfactory Nerves, arising by three roots from the posterior and inferior surface of the anterior lobes, pass forward to the cribriform plate of the ethmoid bone, where they each expand into an oblong body, the olfactory bulb. From its under surface from fifteen to twenty filaments pass downward through the foramina, to be distributed to the olfactory mucous membrane, where they terminate in long, delicate, spindle-shaped cells, the olfactory cells, situated between the ordinary epithelial cells.

The olfactory bulbs are the centers in which odorous impressions are perceived as sensations, destruction of these bulbs being attended by an abolition of the sense of smell.

In animals which possess an acute sense of smell there is a corresponding increase in the development of the olfactory bulbs.

The Essential Conditions for the sense of smell are (1) a special nerve center capable of receiving impressions and transforming them into odorous sensations. (2) Emanations from bodies which are in a gaseous or vaporous condition. (3) The odorous emanations must be drawn freely through the nasal fosse; if the odor be very faint, a peculiar inspiratory movement is made, by which the air is forcibly brought into contact with the olfactory filaments. The secretions of the nasal fosse probably dissolve the odorous particles.

Various substances, as ammonia, horseradish, etc., excite the sensibility of the mucous membrane, which must be distinguished from the perception of true odors.

THE SENSE OF SIGHT.

The Eyeball.-The eyeball, or organ of vision, is situated at the fore part of the orbital cavity and supported by a cushion of fat; it is protected from injury by the bony walls of the cavity, the lids and lashes, and is so

situated as to permit of an extensive range of vision. The eyeball is loosely held in position by a fibrous membrane, the capsule of Tenon, which is attached on the one hand to the eyeball itself and on the other to the walls of the cavity. Thus suspended, the eyeball is capable of being moved in any direction by the contraction of the muscles attached to it.

Structure. The eyeball is spheroidal in shape and measures about ninetenths of an inch in its antero-posterior diameter, and a little less in its transverse diameter. When viewed in profile it is seen to consist of the segments of two spheres, of which the posterior is the larger, occupying five-sixths, and the anterior the smaller, occupying one-sixth of the ball.

The eye is made up of several membranes concentrically arranged, within which are enclosed the refracting media essential to vision. These membranes, enumerated from without inward, are: Ist, the sclerotic and cornea; 2d, the choroid and iris; 3d, the retina; the refracting media are the aqueous humor, the crystalline lens, and vitreous humor.

The Sclerotic and Cornea-The sclerotic is the opaque fibrous membrane covering the posterior five-sixths of the ball. It is composed of connective tissue arranged in layers which run both transversely and longitudinally; it is pierced posteriorly by the optic nerve about one-tenth of an inch internal to the optic axis. The sclerotic by its density gives form to the eye and protects the delicate structures within it, and serves for the attachment of the muscles by which the ball is moved.

The cornea is a transparent non-vascular membrane covering the anterior one-sixth of the eyeball. It is nearly circular in shape and is continuous at the circumference with the sclerotic, from which it cannot be separated. The substance of the cornea is made up of thin layers of delicate, transparent fibrils of connective tissue more or less united together; between these layers are found a number of inter-communicating lymph spaces lined by endothelium, which are in connection with lymphatics. Leucocytes or lymph corpuscles are often found in these spaces. The anterior surface of the cornea is covered by several layers of nucleated epithelium which rest upon a structureless membrane known as the anterior elastic lamina. The posterior surface is covered by a similar membrane, the membrane of Descemet, which becomes continuous at its periphery with the iris; it is also covered by a layer of epithelial cells. At the junction of the cornea and sclerotic is found a circular groove, the canal of Schlemm.

The Choroid, the Iris, the Ciliary Muscle, and Ciliary processes together constitute the second or middle coat of the eyeball.

The choroid is a dark-brown membrane which extends forward nearly